6061-T6 1

[RoarkS]

Okay I got blindsided a bit today.
DER said that 6061-T6 isn't industry standard and everyone wants to get away from it because of fatigue issues.

I tilted my head and said huh?

Thoughts?

 

[rb1957]

I think the DER is used to seeing 2024T3 brkts, so that when he sees one in 6061T6 (or 5082) he may well say "WTF".

With respect to this DER (whom I don't know) this could've been put to bed with a discussion about stress levels, leading to a realisation that the material whilst possibly uncommon in this type of application is quite acceptable. But it sounds to me like the DER has dug his heels in and is saying (my subtext) "if you want me to approve it, make it from 2024T3". Once he's decided that, there's little chance of him changing his mind.

Someone (FAA?) may audit him later and if he had approved it (on the basis of acceptable stress) ask "but the fatigue of 6061T6 is less than 2024T3, how did you account for this ?" or "but the stress corrosion cracking threshold is lower" (if it is). It could "just" be CYA ?

another day in paradise, or is paradise one day closer ?

 

[jaceb]

Hi Tmoose,
Unfortunately it was some time ago and photos has been lost in one of numerous computer migrations. All i can say it is in first case i had lengthy discussion with fameous US bicycle manufacturer (with name starting from S and ending with d) when ive tried to explain to them that they are far from perfection selling welded frames with welds located one on the top of other - instead they tuned down my complains with sending me their user manual for the bicycle owners, after short analysis of the document my conclusion was - according to the manual - all i can do with their bicycle is directly after pourchase, hanging it on the wall in living room and admire it's beauty any other usage is clear violation of manual rules. Remaining two cases with Manufacturer from Poland were straight fwd, they swap frames for new one without any discussion - actually in the first case in the second case they have asked me about distances i'm travelling usually and after ive shared with them this info they just said that i need speciall frame from slightly different model and sice this i'm satisfied custtomer of their company.

 

[drawoh]

thebard3,

I designed a welded aluminium structure many years ago, and I called up 5086. The shop got back to me and they called it "shipping plate". 5086 is excellent stuff to use on ships because it is very corrosion resistant, and it is strong in the annealed condition it will be in after you weld it.

I have seen an electronics enclosure that almost certainly was bent out of 2024[‑]T4. It looked like they cut the aluminium out with a nibbler.

It is not the material itself that matters here. It is the structural analysis safety factor, and using whatever material was specified.

--
JHG

 

[thebard3]

drawoh;
Like I know anything at all about metallurgy! I remember writing the alloy type on a piece I saved, which I still have somewhere. It could be 5086 or anything else, really. When this happened to me, I do recall thinking "They make planes out of this stuff?"
You gotta admit, when your ship is cracking in two in 40 foot seas, that makes for a good story to tell later. Maybe I was just waiting for any excuse to post those pictures here.

Brad Waybright

The more you know, the more you know you don't know.

 

[mfgenggear]

Qoute "drawoh;
Like I know anything at all about metallurgy! I remember writing the alloy type on a piece I saved, which I still have somewhere. It could be 5086 or anything else, really. When this happened to me, I do recall thinking "They make planes out of this stuff?"
You gotta admit, when your ship is cracking in two in 40 foot seas, that makes for a good story to tell later. Maybe I was just waiting for any excuse to post those pictures here." unquote

@thebard3

there could be many reasons the deck was cracking.
with out a failure analysis it's all conjecture. material defective, manufacturing procedures in correct, or improper. in-proper design, (S/N curves), corrosion cracking (in-proper coating)
maybe the forty foot seas caused excessive stress not considered. and many more..

 

[WKTaylor]

MFGE... drifting off-topic...

As I understood it, the Navy learned a bunch of hard lessons about aluminum superstructure [Hulls were still made of steel]. Weight savings and better CG prompted use of aluminum superstructure... but [2] problems arose...

Fracture due to overly stiff connections between the superstructure to the flexible steel hull structure.

Weldable aluminum armor plate performs 'poorly' in an oceanic environment; but the conventional 'ship-hull' aluminum alloys tend to have VERY poor battle-damage/fire durability... IE: ballistic/pyro/explosive penetrators punch thru even thick aluminum hull alloys... and aluminum tends to 'melt-out' at low fire temperatures.

Just for grins...
AWS D3.7 Guide for Aluminum Hull Welding
MIL-HDB-843 SHIP METALLIC MATERIAL COMPARISON



Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[jaceb]

Will,

I thought that Falkland conflict experiences prooved clearly that aluminum superstructures of the warships is bad idea - ship survivability is suffering in unacceptable level - but clearly many navies including USN disregarded this hard lesson.

 

[Tmoose]

Are there corrosion concerns?

I think generically 2024 scores poorly compared to 6061 in corrosion resistance.
And in turn corrosion CLOBBERS a material's fatigue strength.

 

[moon161]

I had a corrosion failure on my 2nd or 3rd generation (3.0 series) cannondale racing bike. The front derailleur mounted to tapped inserts instead of clamping on. Salt and or dissimilar metal corrosion just turned the surrounding metal to crunchy powder. They were 6061 treated to T6 after welding. Vitus used to use 2024 bonded to forged lugs at the joints but I see they now used welded 6061.

 

[rb1957]

my good ol' CrMo street bike (from Williams in Cheltenham) is still going fine ... 30+ years on (sits in the garage more than it should these days ...)

another day in paradise, or is paradise one day closer ?